The application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2001-055109 filed Feb. 28, 2001, the entire contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates a semiconductor laser and a method of manufacturing the semiconductor laser. Particularly, the present invention relates to a semiconductor laser and a method of manufacturing the semiconductor laser which is capable of attaining a high power laser light emission with high efficiency by reducing a reactive current which does not contribute to laser oscillation.
2. Description of the Related Art
Recently, with the progress of optical communications systems, long-distance communications by means of optical communications cables has been realized.
The semiconductor laser to be employed as a light source of the optical communications systems must have characteristics of high efficiency and high power.
FIG. 5 shows a sectional structure of a generally used buried-type semiconductor laser capable of obtaining a high efficiency laser light. The semiconductor laser of this type is conventionally known and disclosed in Jpn. Pat. Appln. No. 7-22691.
Specifically as shown in FIG. 5, in the semiconductor laser, a first clad layer 2 of a p-type InP is formed on a p-type InP substrate 1 having a (100) crystal plane or a crystal plane close to the (100) crystal plane as the upper surface.
On the upper center of the first clad layer 2, a mesa stripe portion 3 having a trapezoidal shape is formed.
Furthermore, outside the mesa stripe portion 3 on the first clad layer 2, a current blocking portion 4 is formed.
The mesa stripe portion 3 is formed of a projecting portion 2a of the first clad layer 2, an active layer 5 of non-doped InGaAsP formed on the projecting portion 2a of the first clad layer 2, and a second clad layer 6 of n-type InP formed on the active layer 5.
The current blocking portion 4 at both sides of the mesa stripe portion 3 is formed of an n-type current blocking layer 7 of n-type InP for blocking migration of holes present at the lower side, and a high resistance semiconductor layer 8 doped with Fe, for blocking migration of electrons present at the upper side.
A third clad layer 9 of n-type InP is formed so as to simultaneously cover the upper surface of the mesa stripe portion 3 and the upper surface of the current blocking portion 4.
On the third clad layer 9, a contact layer 10 is formed.
On the upper surface of the contact layer 10, an insulating layer 11 is formed so as to face the current blocking portion 4.
An electrode plate 12 is attached to the portion of the upper surface of the contact layer 10 facing to the mesa stripe portion 3.
Furthermore, an electrode plate 13 is attached also on the lower surface of the P-type InP substrate 1.
In the semiconductor laser thus constructed, when a direct-current driving voltage is applied across the upper and lower electrode plates 12 and 13, the current is restricted by the current blocking portion 4 due to the presence of the n-type current blocking layer 7 and the high-resistance semiconductor layer 8.
As a result, the current is concentrated on the mesa-stripe portion 3 at the center, increasing the efficiency of laser light emitting from the active layer 5 of the mesa stripe portion 3.
Furthermore, in the semiconductor laser, it is necessary to minimize a reactive current (leakage current) flowing not through the active layer 5 of the mesa stripe portion 3 but from the second clad layer 6 to the n-type current blocking layer 7.
To avoid direct contact between the first clad layer 2 (2a) formed of p-type InP and the high-resistance semiconductor layer 8, a top-end 7a of the n-type current blocking layer 7 is positioned on the border between the active layer 5 and the second clad layer 6.
To form such a structure, etching is performed in its manufacturing process of the semiconductor laser in the conditions under which a (111)B crystal plane can be exposed on an inclined side surface 14 of the mesa stripe portion 3 having a trapezoidal shape.
Furthermore, a (100) crystal plane is exposed by etching on the upper surface 15 of the first clad layer 2 outside the mesa stripe portion 3.
Thereafter, the n-type current blocking layer 7 is grown on the inclined side surface 14 of the mesa stripe portion 3 and on the upper surface 15 of the first clad layer 2 by use of a metal-organic-vapor-phase epitaxy (MOVPE) method.
As known well, the n-type current blocking layer 7 is grown directly on the (100) crystal plane but not grown directly on the (111)B crystal plane.
Accordingly, in the case where the n-type current blocking layer 7 is grown by use of the metal-organic-vapor-phase epitaxy (MOVPE) method, a tapered tip 7a of the n-type current blocking layer 7 creeps up along the inclined side surface 14 of the mesa stripe portion 3 in accordance with the growth of the n-type current blocking layer 7, as shown in FIG. 6.
Accordingly, when the tapered tip 7a of the n-type current blocking layer 7 reaches the border between the active layer 5 and the second clad layer 6, the growth operation of the n-type current blocking layer 7 by use of the metal-organic-vapor-phase epitaxy (MOVPE) method is terminated The manufacturing method mentioned above makes it possible to minimize the reactive current (leakage current) flowing from the second clad layer 6 to the n-type current blocking layer 7 without passing through the active layer 5 of the mesa stripe portion 3.
Furthermore, by employing the Fe-doped high resistant semiconductor layer 8 as the current blocking portion 4, a high-speed operation can be attained.
However, the conventional semiconductor laser having the structure shown in FIG. 5 still have the following problems to be solved.
In the case where the n-type current blocking layer 7 is grown by the metal-organic-vapor-phase epitaxy (MOVPE) method as shown in FIG. 6, the tapered tip 7a of the n-type current blocking layer 7 creeps up along the inclined side surface 14 of the mesa stripe portion 3 as it grows.
Thereafter, when the tapered tip 7a reaches the border between the active layer 5 and the second clad layer 6, it is necessary to terminate the growth operation using the metal-organic-vapor-phase epitaxy (MOVPE) method.
However, the timing (time) at which the tapered tip 7a reaches the border between the active layer 5 and the second clad layer 6 varies depending upon voltage application conditions of the metal-organic-vapor-phase epitaxy (MOVPE) method and the height of the trapezoidal mesa stripe portion 3 which slightly varies depending upon etching conditions.
Therefore, the tapered tip 7a of the n-type current blocking layer 7 fails to reach the border between the active layer 5 and the second clad layer 6 in some cases, and in other cases, it reaches up to the middle of the side surface of the second clad layer 6.
As a result, the first clad layer 2 (2a) of p-type InP may be brought into direct contact with the high resistance semiconductor layer 8. Alternatively, the amount of the reactive current (leakage current) flowing from the second clad layer 6 to the n-type current blocking layer 7 may increase, with the result that stable and high efficient laser-emitting characteristics as the semiconductor laser cannot be obtained.
Furthermore, in the conventional semiconductor laser mentioned above, a p-type InP substrate is employed.
The p-type InP substrate, as is known well, has a high specific resistance compared to an n-type InP substrate.
As a result, if the amount of a current to be supplied to the semiconductor laser is increased in order to obtain a high-power laser, the generation of heat increases.
Therefore, the semiconductor laser employing the p-type InP substrate is useful in the cases where a switching operation is carried out at a high speed, however is not suitable as a semiconductor laser like a light source for an optical communications system.
An impurity, Zn, diffuses within the p-type semiconductor more easily than within the n-type semiconductor. Therefore, when Zn is doped into a p-type InP substrate as an impurity, the diffusion of Zn is easily performed. It follows that Zn diffuses up to the area in the proximity of the active layer of the p-type InP first clad layer.
Consequently, the laser generation efficiency at the active layer is reduced and thus high power laser light emission cannot be obtained.
As described above, in the semiconductor laser employing the p-type InP substrate, stable and high power laser emission characteristics cannot be obtained.
As disclosed in Jpn. Pat. Appln. No. 7-162078, a semiconductor laser employing an n-type InP substrate in place of a p-type InP substrate is also proposed.
However, in this publication (Jpn. Pat. Appln. No. 7-162078) disclosing a semiconductor laser employing an n-type InP substrate, no mention is made of a structure of the contact portion between the side surface of the mesa stripe portion and the current blocking portion (in contact with the side surface), at all.
In short, as described above, since the amount of the reactive current (leakage current) not flowing through the active layer increases, stable and high efficiency laser emission characteristics as a semiconductor laser cannot be obtained.
The present invention is made in view of the aforementioned circumstances and an object of the present invention is to provide a semiconductor laser and a method of manufacturing the semiconductor laser which is formed by using an n-type semiconductor substrate and setting the inclination angle of the side surface of the mesa strip portion such that a (111)B crystal plane is not directly exposed in the inclined surface of the mesa stripe portion having a trapezoidal shape so as to control a reactive current not flowing through an active layer of the mesa stripe portion to fall within a predetermined range and to maintain a high withstand voltage at the current blocking portion, thereby attaining a high efficiency and high power laser light emission characteristics.
To attain the aforementioned object, according to a first aspect of the present invention, there is provided a semiconductor laser comprising:
an n-type semiconductor substrate having a (100) crystal plane as an upper surface;
a mesa stripe portion having a trapezoidal shape and formed along a  less than 011 greater than  direction, the mesa stripe portion including an n-type first clad layer, an active layer, and a p-type second clad layer, which are successively formed on the n-type semiconductor substrate;
a current blocking portion formed of a p-type current blocking layer formed outside the mesa stripe portion and on the n-type semiconductor substrate, and an n-type current blocking layer is formed on the p-type current blocking layer; and
a p-type third clad layer simultaneously covering both the upper surface of the mesa stripe portion and the upper surface of the current blocking portion,
in which an inclination angle being an acute angle of a side surface of the mesa stripe portion having a trapezoidal shape and formed along a  less than 011 greater than  direction is close to an inclination angle of a (111)B crystal plane with respect to the (100) crystal plane and set at one of an angle larger than and an angle smaller than the inclination angle of the (111)B crystal angle.
According to a second aspect of the present invention, there is provided the semiconductor laser according to the first aspect in which
a thickness of the p-type current blocking layer in a direction perpendicular to an inclined side surface of the mesa stripe portion near the active layer is set to be thinner than a thickness in a direction perpendicular to the (100) crystal plane of the n-type semiconductor substrate; and
an impurity concentration of the p-type current blocking layer near the active layer of the mesa stripe portion is set to be lower than an impurity concentration of the p-type current blocking layer near the (100) crystal plane of the n-type semiconductor substrate.
According to a third aspect of the present invention, there is provided the semiconductor laser according to the second aspect, in which
the n-type semiconductor substrate is formed of n-type InP; and
the p-type current blocking layer is formed of p-type InP containing an impurity of Zn or Cd.
According to a fourth aspect of the present invention, there is provided the semiconductor laser according to any one of the first to third aspects, in which inclination of the side surface of the mesa stripe portion is set at an angle within (+1xc2x0 to +5xc2x0) or (xe2x88x921xc2x0 to xe2x88x925xc2x0) to the inclination angle of the (111)B crystal plane.
According to a fifth aspect of the present invention, there is provided a semiconductor laser according to any one of the first to fourth aspects, in which an upper end of the side surface of the mesa stripe portion is positioned higher than a vicinity of the active layer.
To attain the aforementioned object, according to a sixth aspect of the present invention, there is provided a method of manufacturing a semiconductor laser comprising:
preparing an n-type semiconductor substrate having a (100) crystal plane as an upper surface;
stacking an n-type first clad layer, an active layer, and a p-type second clad layer sequentially on the n-type semiconductor substrate, and, forming a mesa stripe portion having a trapezoidal shape along a  less than 011 greater than  direction, the mesa stripe portion including the n-type first clad layer, the active layer, and a p-type second clad layer;
forming an p-type current blocking layer outside the mesa stripe portion and on the n-type semiconductor substrate and an n-type current blocking layer on the p-type current blocking layer, as the current blocking portion using a metal organic vapor phase epitaxy (MOVPE) method; and
covering both an upper surface of the mesa stripe portion and an upper surface of the current blocking portion with a p-type third clad layer,
in which an inclination angle being an acute angle of the side surface of the mesa stripe portion having a trapezoidal shape and formed along the  less than 011 greater than  direction is close to an inclination angle of a (111)B crystal plane with respect to the (100) crystal plane and set at one of an angle larger than and an angle smaller than an inclination angle of the (111)B crystal plane.
According to a seventh aspect of the present invention, there is provided a method of manufacturing a semiconductor laser according to a sixth aspect,
in which the mesa stripe portion is formed by forming a mask on an upper surface of the second clad layer and etching the n-type first clad layer, the active layer, and the p-type second clad layer into a trapezoidal shape;
the current blocking portion is formed by growing the p-type current blocking layer and the n-type current blocking layer on the side surface of the mesa stripe portion and on the upper surface of the n-type semiconductor substrate exposed by etching;
the p-type third clad layer is formed so as to simultaneously cover both the upper surface of the mesa stripe portion from which the mask is removed and the upper surface of the current blocking portion;
forming the mesa stripe portion comprises
interposing a cap layer between the second clad layer and the mask;
setting conditions of etching such that inclination of the side surface of the mesa stripe portion to be obtained by side etching of the cap layer has an angle near a predetermined angle at which the (111)B crystal plane is exposed, excluding the predetermined angle; and
removing the mask simultaneously with the cap layer.
According to an eighth aspect of the present invention, there is provided the method of manufacturing a semiconductor laser according to the sixth aspect, in which the n-type semiconductor substrate is InP doped with an n-type impurity, the first clad layer is InP doped with an n-type impurity, the active layer has a multiple quantum well structure consisting of non-doped InGaAs, non-doped InGaAsP, or a combination thereof, the second clad layer is InP doped with a p-type impurity, the cap layer is InGaAsP, and the p-type current blocking layer is p-type InP doped with an impurity of Zn or Cd.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.